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<td> <div>定义于头文件 <code><a href="../header/bit.html" title="cpp/header/bit">&lt;bit&gt;</a></code>
 </div></td>
<td></td>
<td></td>
</tr>
<tr class="t-dcl t-since-cxx20">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span> To, <span class="kw1">class</span> From <span class="sy1">&gt;</span><br>
<span class="kw4">constexpr</span> To bit_cast<span class="br0">(</span><span class="kw4">const</span> From<span class="sy3">&amp;</span> from<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-since-cxx20">(C++20 起)</span> </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>通过转译 <code>from</code> 的对象表示获得 <code>To</code> 类型值。返回的 <code>To</code> 对象值表示中的每位都等于 <code>from</code> 的对象表示中的对应位。返回的 <code>To</code> 对象中的填充位的值未指定。
</p><p>若无 <code>To</code> 类型值对应产生的值表示，则行为未定义。若有多个这种值，则产生哪个值是未指定的。
</p><p>此重载仅若 <span class="t-c"><span class="mw-geshi cpp source-cpp">sizeof<span class="br0">(</span>To<span class="br0">)</span> <span class="sy1">==</span> sizeof<span class="br0">(</span>From<span class="br0">)</span></span></span> 且 <code>To</code> 和 <code>From</code> 两者都是<a href="../named_req/TriviallyCopyable.html" title="cpp/named req/TriviallyCopyable"><i>可平凡复制</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(TriviallyCopyable)</span> </a>类型才参与重载决议。
</p><p>此函数模板为 <code>constexpr</code> ，若且唯若 <code>To</code> 、 <code>From</code> 和 <code>To</code> 及 <code>From</code> 的所有子对象的类型都：
</p>
<ul><li> 不是联合体类型；
</li><li> 不是指针类型；
</li><li> 不是指向成员指针类型；
</li><li> 不是 volatile 限定类型；且
</li><li> 无引用类型的非静态数据成员。
</li></ul>
<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
<p>值表示描述如上的 <code>To</code> 类型对象。
</p>
<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
<p>大多数情况下不应该用指针或引用类型间的 <a href="../language/reinterpret_cast.html" title="cpp/language/reinterpret cast"><tt>reinterpret_cast</tt></a> （或等价的<a href="../language/explicit_cast.html" title="cpp/language/explicit cast">显式转型</a>）转译对象表示，因为<a href="../language/reinterpret_cast.html#.E7.B1.BB.E5.9E.8B.E5.88.AB.E5.90.8D.E4.BD.BF.E7.94.A8" title="cpp/language/reinterpret cast">类型别名使用规则</a>。
</p><p>在 <code>std::bit_cast</code> 前，需要翻译对象表示为另一类型的对象表示时，能使用 <span class="t-lc"><a href="../string/byte/memcpy.html" title="cpp/string/byte/memcpy">std::memcpy</a></span> ：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> To, <span class="kw1">class</span> From<span class="sy1">&gt;</span>
<span class="kw1">typename</span> <a href="../types/enable_if.html"><span class="kw630">std::<span class="me2">enable_if</span></span></a><span class="sy1">&lt;</span>
    <span class="br0">(</span>sizeof<span class="br0">(</span>To<span class="br0">)</span> <span class="sy1">==</span> sizeof<span class="br0">(</span>From<span class="br0">)</span><span class="br0">)</span> <span class="sy3">&amp;&amp;</span>
    <a href="../types/is_trivially_copyable.html"><span class="kw500">std::<span class="me2">is_trivially_copyable</span></span></a><span class="sy1">&lt;</span>From<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span> <span class="sy3">&amp;&amp;</span>
    <a href="../types/is_trivial.html"><span class="kw498">std::<span class="me2">is_trivial</span></span></a><span class="sy1">&lt;</span>To<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span>,
    <span class="co1">// 此实现要求 To 可平凡默认构造</span>
    To<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">type</span>
<span class="co1">// constexpr 支持需要编译器魔法</span>
bit_cast<span class="br0">(</span><span class="kw4">const</span> From <span class="sy3">&amp;</span>src<span class="br0">)</span> <span class="kw1">noexcept</span>
<span class="br0">{</span>
    To dst<span class="sy4">;</span>
    <a href="../string/byte/memcpy.html"><span class="kw1159">std::<span class="me2">memcpy</span></span></a><span class="br0">(</span><span class="sy3">&amp;</span>dst, <span class="sy3">&amp;</span>src, sizeof<span class="br0">(</span>To<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="kw1">return</span> dst<span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;cstdint&gt;</span>
<span class="co2">#include &lt;bit&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
 
<span class="kw4">constexpr</span> <span class="kw4">double</span> f64v <span class="sy1">=</span> <span class="nu16">19880124.0</span><span class="sy4">;</span>
<span class="kw4">constexpr</span> <span class="kw4">auto</span> u64v <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">bit_cast</span><span class="sy1">&lt;</span><a href="../types/integer.html"><span class="kw123">std::<span class="me2">uint64_t</span></span></a><span class="sy1">&gt;</span><span class="br0">(</span>f64v<span class="br0">)</span><span class="sy4">;</span>
 
<span class="kw4">constexpr</span> <a href="../types/integer.html"><span class="kw123">std::<span class="me2">uint64_t</span></span></a> u64v2 <span class="sy1">=</span> 0x3fe9000000000000ull<span class="sy4">;</span>
<span class="kw4">constexpr</span> <span class="kw4">auto</span> f64v2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">bit_cast</span><span class="sy1">&lt;</span><span class="kw4">double</span><span class="sy1">&gt;</span><span class="br0">(</span>u64v2<span class="br0">)</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a>
        <span class="sy1">&lt;&lt;</span> <a href="../io/manip/fixed.html"><span class="kw1784">std::<span class="me2">fixed</span></span></a> <span class="sy1">&lt;&lt;</span>f64v <span class="sy1">&lt;&lt;</span> <span class="st0">"f64.to_bits() == 0x"</span>
        <span class="sy1">&lt;&lt;</span> <a href="../io/manip/hex.html"><span class="kw1782">std::<span class="me2">hex</span></span></a> <span class="sy1">&lt;&lt;</span> u64v <span class="sy1">&lt;&lt;</span> <span class="st0">"u64<span class="es1">\n</span>"</span><span class="sy4">;</span>
 
    <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a>
        <span class="sy1">&lt;&lt;</span> <span class="st0">"f64::from_bits(0x"</span> <span class="sy1">&lt;&lt;</span> <a href="../io/manip/hex.html"><span class="kw1782">std::<span class="me2">hex</span></span></a> <span class="sy1">&lt;&lt;</span> u64v2 <span class="sy1">&lt;&lt;</span> <span class="st0">"u64) == "</span>
        <span class="sy1">&lt;&lt;</span> <a href="../io/manip/fixed.html"><span class="kw1784">std::<span class="me2">fixed</span></span></a> <span class="sy1">&lt;&lt;</span> f64v2 <span class="sy1">&lt;&lt;</span> <span class="st0">"f64<span class="es1">\n</span>"</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>可能的输出：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">19880124.000000f64.to_bits() == 0x4172f58bc0000000u64
f64::from_bits(0x3fe9000000000000u64) == 0.781250f64</pre></div></div> 
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